This invention relates to the problem of preparing random terpolymers of ethylene, propylene and a diene, such as ethylidenenorbornene (ENB), dicyclopentadiene (DCPD), or 1,4-hexadiene (HD). These terpolymers are known as EPDM and can be fabricated and cured like many other natural and synthetic rubbers.
In commercial practice, EPDM is prepared in a Ziegler-Natta polymerization process that uses a catalyst system which comprises a Group IV-Group VII transition metal compound, e.g., a vanadium compound, as one component of the catalyst and a Group I-III organometallic compound, e.g., an alkylaluminum halide, as the other component of the catalyst. (See, for example, The Encyclopedia of Polymer Science and Engineering, Vol. 6 (1986) at pp. 545-546 and U.S. Pat. No. 4,347,159). Vanadium compounds used for this process include VOCl.sub.3, VCL.sub.4, V(2,4-pentanedionate).sub.3, and simple derivatives of these compounds. One shortcoming of the existing systems is that they show a low activity for incorporating the diene and propylene relative to their activity for ethylene. This results in the use of excess propylene and diene in the polymerization process, which, in order to obtain the desired level of incorporation, must be recycled. Eliminating or reducing this need for recycling excess diene would be useful.